The art of fiber-reinforced composite (resin) vehicle wheel manufacture, as taught for example by U.S. Pat. Nos. 4,294,490 and 4,294,639, is now sufficiently advanced that such wheels are superior in many respects--e.g., corrosion-resistance, fuel economy and durability--to conventional steel and aluminum-alloy wheels. However, a problem that has continued to plague the art lies in securely mounting such wheels to a vehicle, specifically in maintaining desired fastener torque clamping tension in the wheel-mounting studs. The resin in the composite wheel structure has exhibited a tendency to creep under high clamping stress, particularly when subjected to high temperatures during severe brake-cycle vehicle tests.
One early approach to the fastener torque-retention problem, exemplified in U.S. Pat. Nos. 3,309,843, 3,811,737 and 3,870,372, contemplates provision of metallic inserts in the wheel-mounting openings for absorbing fastener compressive stresses. However, this technique involves placement and control of insert location during the molding operation, or additional post-molding operations to add the inserts, both with consequent additional costs. Moreover, control of fiber orientation during the molding operation presents a problem where the inserts are molded in place. Whether the inserts are placed in the wheel during the molding operation or in a subsequent machining and fastening operation, there is the possibility of separation of the insert from the surrounding fiber-reinforced composite wheel body. Moreover, as will be noted in connection with experimental test results in the following description, such inserts do not overcome the torque-retention problem.
U.S. Pat. Nos. 4,489,184 and 4,569,957 discloses a resin composition that exhibits enhanced temperature/stress stability, and is suitable for use in fiber-reinforced resin vehicle wheel manufacture to overcome the problem of resin creep around the wheel mounting studs. However, it has been found that the reinforcing fibers themselves present a problem that must be addressed. Specifically, where it has been attempted to form the mounting openings during the molding operation, it has been found that the mold plugs cause uneven distribution and orientation of reinforcing fibers around the mounting openings, sometimes resulting in crack formation in under-reinforced portions of the wheel disc. Thus, as disclosed in U.S. Pat. Nos. 4,294,490 and 4,294,639, it is preferable to form the wheel-mounting disc as an imperforate structure during the molding operation in which distribution and orientation of reinforcing fibers can be closely controlled, and to thereafter form the mounting openings in a machining (drilling) operation.
However, such machining of the disc body necessarily severs reinforcing fibers at the surface of the opening, so that fiber ends are exposed at the surface around each mounting opening against which the fastener nut must seat. It has been found that such fiber ends frictionally resist sliding of the opposing nut surface as the nut is tightened. As is well known in the fastener art, torque applied to the mounting nut during tightening is a function both of tension applied to the stud and of sliding friction between the nut and the underlying surface. Where the sliding surfaces exhibit a high coefficient of friction, as in the case where the fastener nut must be tightened against a wheel surface at which reinforcing fiber ends are exposed, tightening the mounting nut to a specified torque as is conventional on the art does not place sufficient tension on the mounting stud to retain clamping pressure and mounting torque during vehicle operation. That is, in mounting wheels of the subject character, the coefficient of friction between the nuts and wheel seating surfaces is excessive, which reduces the tension applied to the mounting studs during the initial tightening operation. For this reason, the wheel mounting nuts typically retain substantially less than 50% of the initial fastening torque after high temperature torque-retention tests.
It is therefore a general object of the present invention to provide a wheel mounting arrangement and method that overcome the aforementioned deficiencies in the art, that are economical to implement in mass production of vehicle wheels and mounting thereof on vehicles, that can be readily implemented in the field during replacement of vehicle wheels, that employ conical nuts of the configuration that is accepted as industry standard for mounting aluminum alloy wheels, and that retains initial fastening torque at a level comparable to that exhibited by conventional steel and aluminum alloy wheels.
Another and related object of the invention is to provide a fiber-reinforced resin vehicle wheel and method of manufacture that are specifically adapted to overcome the aforementioned problems in the art during mounting and use thereof on a vehicle without requiring any intervention or steps by the person mounting the wheel on the vehicle other than those otherwise conventionally employed.